1. Field Of The Invention
This invention relates to the field of stress relieving structures in the metal fabricating industry, such as welded assemblies and more particularly where such relief is obtained by vibration of an assembled structure to arrive at a stable structure substantially free of internal stresses.
2. Description Of The Prior Art
The metalworking industry has experienced considerable difficulty in manufacturing dimensionally accurate heavy industry components such as heavy machine tools, large farm equipment, transportation equipment, construction equipment and various industrial processing machinery, or equipment.
Product quality has improved, but complexity of design has increased and sensitivity to dimensional instability has become correspondingly more acute. One reason for such difficulties in maintaining the dimensional quality has been that the fabrication methods, whether welding, casting, or forging, utilize heat processing of the metal structures. In forming metal it frequently received large quantities of heat to obtain the near molten state required for shaping processes. Such methods produced great temperature differences in the component structures and this causes residual stresses which remained locked in the component structures after the forming or shaping is completed.
It was necessary to reduce or relieve these built-in stresses by loading the completed structures in a complex manner, or by machining, which often removed metal that had at least partially opposed certain of the residual stresses, or by a stress relief treatment such as by annealing the entire component assembly. If machining occurred prior to relieving such residual stresses, warping, twisting, or other dimensional distortion often resulted.
One solution to this problem was the early practice of storing completed workpieces out of doors in all kinds of weather so that the variations in the weather imposed loads, such as those induced by expansion and contraction. This experience often provides sufficient loading and unloading of the workpieces to arrive at some relief of the residual stresses. However, where large fabricated components were involved, the period of stress relief was very extended and might run a year, or two years, or more.
Another method of solving the problem was developed as a means of saving production time and to meet the inventory pressures. This method utilized as an alternative to the storage system, involved a thermal stress relief process in which the fabricated steel component was placed in a furnace and the temperature raised to approximately 1100.degree. F. This temperature was maintained for a period of time that was identified as the soaking period and then it was necessary to resort to a gradual cool down period. While not as lengthy as the storage method, this system also required a considerable period of time to complete properly.
During the process of thermal stress relief, the relation between stress and strain is altered so that the yield point of the material is substantially lowered which allows stresses above the new yield point to cause plastic flow and thereby reduce the level of the residual stresses. This occurs during the soaking period in the thermal stress relief system, but during the cool down the original yield point is re-established with the result that the high level stresses have been reduced and these typically are the residual stresses that interfere with dimensional stability. This method allowed somewhat faster and more consistent processing of dimensionally critical components but like practically all industrial techniques, it had its disadvantages and limitations.
The thermal treatment caused scaling and sagging of the workpiece. This required the extra processing step of removing the scaling before the component could be utilized in production. The heat of the process resulted in the strength of the component being lowered while in the furnace and frequently sagging of the component resulted, frequently because of the very weight of some heavy components which acted in this manner because of their weight. In attempts to avoid this difficulty, braces sometimes were welded across the sag lines, but again this caused additional labor and material expense.
Frequently, metallurgical changes occurred in a component that altered the physical characteristics of the material and which was usually negative. A number of metals react in this manner.
The energy requirements of the thermal process, especially where a large furnace must be utilized for very large components, is enormous and where heavy wall thicknesses are utilized in the plate structures of the components a greater period of treatment is necessitated with consequently greater cool down time, all of which contribute greatly to the expense of this system.
A prior art method of stress relieving a work piece by vibration is disclosed in U.S. Pat. No. 3,622,404, but this method required vibration of a workpiece in the frequency range of the resonant peak for each part of the piece to be relieved and maintaining the vibration in the frequency range of each such peak while the amplitude of the peak increases and the power to produce the peak decreases while the frequency range decreases until the power producing the amplitude has stabilized.
However, the acceleration data was distorted because the accelerometer developed resonance within the range under study. Also, poor filtering in the control console affected the acceleration signal and the acceleration data was not completely, or properly presented to the operator so that he could detect the treatment frequencies. A meter was used to indicate resonance. The arrangement lacked an electronic motor speed control and therefore the motor speed accuracy was poor primarily because only a voltage control was used and not any form of negative feedback. The vibrator used with this prior method had an output of 2 or 3 inch pounds so that the vibrator in service often had too little force output to accomplish the job.